Heave compensator

ABSTRACT

A heave compensator for a floating vessel maintains a substantially constant load on a coring or drilling tool at the bottom of a drill string supported by the vessel. The heave compensator includes a displacement compensating cylinder having an adjustable volume of hydraulic fluid presented to a piston in the cylinder and supporting the upper end of the drill string. The volume displacement of fluid in the cylinder varies in response to the heave of the vessel to maintain the drill string in a substantially fixed position independent of the heave of the vessel; this, in turn, provides a substantially constant load on the tool. The displacement compensating cylinder is hydraulically linked to one side of a servo ram in a hydraulic accumulator for accommodating fluid from the displacement compensating cylinder. The opposite servo side of the servo ram is initially buffered by a passive air bank when the load on the tool is within narrow plus or minus limits. When the load exceeds these limits, a hydraulic servo system linked to the servo ram actively assists the passive air bank in compensating for the excessive load excursion. The servo system includes means for sensing predetermined excursions in the load on the tool, and a control valve which either presents hydraulic fluid from a pump to the servo ram, or vents hydraulic fluid from the ram in response to operation of the sensing means when the sensed load reaches its predetermined limits.

Unite States Hooper atent I 1 [4 1 Sept. 16, 1975 HEAVE COMPENSATOR [75] Inventor: David William Hooper, La Habra,

Calif.

[73] Assignee: Global Marine Inc., Los Angeles,

Calif.

[22] Filed: Oct. 9, 1973 211 Appl. No.: 404,617

[52] US. Cl. 254/172; 91/390; 60/413; 92/108; 175/27 [51] Int. Cl B66d l/48 [58] Field of Search 254/172, 173 B, 189; 91/4, 91/390, 359, 189, 364, 433; 60/413, 418; 267/125; 175/27; 92/110, 108

Primary ExuminerRobert J. Spar Assistant Examinerl(enneth Noland Attorney, Agent, or Firm-Christie, Parker & Hale 57 ABSTRACT A heave compensator for a floating vessel maintains a substantially constant load on a coring or drilling tool at the bottom of a drill string supported by the vessel. The heave compensator includes a displacement compensating cylinder having an adjustable volume of hydraulic fluid presented to a piston in the cylinder and supporting the upper end of the drill string. The volume displacement of fluid in the cylinder varies in response to the heave of the vessel to maintain the drill string in a substantially fixed position independent of the heave of the vessel; this, in turn, provides a substantially constant load on the tool. The displacement compensating cylinder is hydraulically linked to one side of a servo ram in a hydraulic accumulator for accommodating fluid from the displacement compensating cylinder. The opposite servo side of the servo ram is initially buffered by a passive air bank when the load on the tool is Within narrow plus or minus limits. When the load exceeds these limits, a hydraulic servo system linked to the servo ram actively assists the passive air bank in compensating for the excessive load excursion. The servo system includes means for sensing predetermined excursions in the load on the tool, and a control valve which either presents hydraulic fluid from a pump to the servo ram, or vents hydraulic fluid from the ram in response to operation of the sensing means when the sensed load reaches its predetermined limits.

HEAVE COMPENSATOR BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to heave compensators which maintain a substantially constant load on or position of an article suspended from a floating vessel during heaving motion of the vessel. More particularly, this invention relates to a hydraulic servo system for actively assisting a passive pneumatic bias circuit in maintaining the load on or position of the article within predetermined narrow limits throughout high amplitudes of heave motion of the vessel.

2. Description of the Prior Art During underwater coring or well drilling operations carried out from a floating vessel, such as a drill ship, the vessel if often subjected to upward and downward heave motions due to wave action. A coring or drilling tool, for example, is carried on the lower end of a string of drill pipe which is pendulously supported on the vessel. In coring operations uncompensated heaving motion of the vessel can cause substantial variations in the force applied by the coring tool to the underwater formation, which force variations produce compactions or weakening of the core and thereby destroy or substantially reduce the usefulness of the core for its intended purposes. During drilling operations heave-induced load variations on a drill bit produce excessive wear of the bit.

Heave compensators are known for maintaining a substantially constant force on the coring or drilling tool independent of the heave of the vessel. The known heave compensators generally include a motioncompensating cylinder and piston assembly associated with the crown block or the travelling block of a derrick located on the vessel and from which the drill string is suspended. The cylinder is charged with a volume of hydraulic fluid which flows in and out of the cylinder in response to the downward and upward heave motions of the vessel. The flow of hydraulic fluid from and to the cylinder is to and from a pneumo-hydraulic accumulator which is, in turn, charged with compressed air from a bank of compressed air bottles or the like on the vessel. The known heave compensators are purely passive in nature. The nominal pressure charge of the air bank, and thereby of the accumulator, determines the nominal hydraulic pressure in the cylinder. The cylinder hydraulic pressure in turn determines the value ofthe load which is to be suspended from the vessel and which is to be held substantially constant despite heaving motion of the vessel.

Due to practical limitations of weight and size, the capacity of the air (bottle) bank is not infinite. Therefore, flow of hydraulic fluid to and from the accumulator in response to heave motion of the vessel produces increases and decreases in the accumulator air pressure relative to the nominal charged air pressure; these variations are manifested as corresponding variations in the loads suspended from the vessel, i.e., variations in the portion of the total weight of the drill string and coring or drilling tool borne by the submerged formation engaged by the tool. The purely passive known heave compensators can maintain the load on the coring of drilling bit constant to within plus or minus about seven percent, at best, of a preset load value in the presence of large heave amplitudes of the vessel. A need exists for a heave compensator arrangement which can maintain the load on the coring or drill bit constant to within plus or minus two percent or so of a preset load value.

SUMMARY OF THE INVENTION This invention provides a heave compensator which is effective to maintain constant, within plus or minus two percent or so, the load on a coring or drill bit engaged with an underwater earth formation, and suspended from a floating vessel subject to high amplitude heave motions.

Broadly summarized, this invention provides a load sensitive heave compensating apparatus which is useful to maintain a substantially constant support force on an article suspended from a floating vessel by a vertical support member despite heaving movement of the vessel in response to wave action and the like. The heave compensator apparatus includes displaceable means engageable between the vessel and the support member at the vessel. The displaceable means is operable for varying the distance between the vessel and the article. The apparatus also includes passive means which are coupled to the displaceable means for operating the displaceable means to resiliently establish in the support member a desired load which corresponds to a desired value of support force for the article. The passive means are also operative to maintain the desired load constant within preselected limits of variation of the desired load. The heave compensator apparatus further includes active means coupled to the displaceable means and responsive to predetermined excursions in the desired load for actively assisting the passive means to maintain the desired load within said preselected limits.

In the context of the preceding remarks, the support member may be a string of drill pipe which extends from the vessel to an underwater geological formation. The article may be a hole forming tool such as a coring tool or drilling tool, carried by the lower end of the drill string. The suppot force may be an upwardly acting force applied by the formation to the tool in response to engagement of the tool with the formation.

The foregoing summary contemplates that the support force on the article which is to be maintained constant within narrow limits may be the weight of the article itself, as when the article is a blow-out preventer stack being lowered into engagement with a wellhead structure for an exploration or production oil or gas well. Also, the support member may be a cable or a string of drill pipe as mentioned above. The displaceable means referred to in the preceding summary may be a linear action device, or rotary action device such as a winch. Where the displaceable means is-a linear action mechanism, the mechanism may or may not be vertically disposed. Also, the displaceable means may be connected between the vessel and the upper end of the support member, or it may be connected from the vessel to the support member at some point along the length of the support member. The desired load established by the passive means may be other than the constant support force to be maintained on the article, as when a mechanical advantage exists between the displaceable means and the article. Further, the underwater geological formation may be a silt or mud layer on or immediately below the ocean floor or it may be a mineral stratum located some distance below the ocean floor.

The present improved heave compensator apparatus, as opposed to the purely passive devices described above, is a passive-active/passive apparatus in that low amplitude heave motions are compensated for by a purely passive aspect of the apparatus, whereas higher amplitude heave motions are compensated for by the operation of an active mechanism in combination with the passive aspect of the apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS These and other aspects of the invention will be more fully understood by referring to the following detailed description of a presently preferred embodiment of the invention and to the accompanying drawings in which:

FIG. 1 is a fragmentary elevation view showing the preferred heave compensating system connected to the upper end of a drill string extending downwardly from a floating vessel to an underwater formation; and

FIG. 2 is a schematic diagram of the heave compensating system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 shows a core hole being formed in an underwater formation 12 from a drilling vessel 14 floating on a body of water 16. An upright, hollow, elongated string of drill pipe 18 extends downwardly from vessel 14 through the body of water to core hole 10. A coring tool 20 is affixed to the lower end of drill string 18. The top section of drill string 18 is passed through a well 22 which opens centrally through vessel 14. A derrick 24 mounted on the deck of vessel 14 supports a stationary crown block 25 above center well 22. A travelling block 26 is suspended from the end of a cable 28 extending over crown block 25 from a winch drum 32 mounted onboard the vessel. The drill string is passed through a conventional rotary table 21 mounted on the vessel above well 22. The rotary table is operable to rotate the drill string, which rotation is relied on to produce operation of the coring tool.

Travelling block 26 supports a vertically extending displacement compensating cylinder 34. a piston 36 (hereinafter sometimes called the hook piston or the compensator piston) in cylinder 34 carries a hook 38 which supports a swivel 40 engaged with the top of drill string 18.

Displacement compensating cylinder 34 is part of a heave compensation system 35 (see FIG. 2) of this invention which maintains a constant load on coring tool 20 when vessel 14 heaves upward or downward in response to wave action. The present invention will be described in the context of the rotary core forming system of FIG. 1 for illustrative purposes only. However, it will be understood from the following detailed description that the present invention also can be used to control load variations on a punch coring tool or on a rotary drill bit of the type used to drill oil or gas wells.

Heave compensation system 35 will be understood best by referring to FIG. 2. The underside of hook piston 36 is supported by an adjustable volume 37 of hydraulic fluid in compensation cylinder 34. The fluid flows in and out of cylinder 34 in response to the downward and upward heave, respectively, of vessel 14. In FIG. 1, the distance Ad, represents upward heaveinducing excursion of the water which causes the vessel to move upward a distance AV Ad represents a downward heave-inducing excursion of the water which causes the vessel to move downward a distance AV AV and AV may have magnitudes of ten feet or so. Cylinder 34, which is connected to the vessel via travelling block 26, moves up and down as the vessel heaves, while piston 36 remains essentially free floating on the variable volume of hydraulic fluid below it. This maintains the piston in an essentially constant position independent of the heave of the vessel. Thus, the distance between piston 36 and the bottom of the core hole remains essentially constant independent of the heave of the vessel, which thereby maintains a constant load on coring tool 20. Piston 36 has a stroke in cylinder 34 which is at least equal to the sum of AV and AV which can be tolerated before heaving motions become so severe as to require that coring operations be discontinued for the safety of the personnel involved.

The inward and outward flow of hydraulic fluid from the lower section of cylinder 34 is accommodated by a pneumo-hydraulic servo accumulator 42 hydraulically linked to cylinder 34 and volume 37 thereof via a flexible hose 44. As illustrated in FIG. 1, accumulator 42 is rigidly connected to derrick 24, and hose 44 accommodates any upward or downward movement of cylinder 34 relative to accumulator 42.

A servo ram 45 in accumulator 42 includes a slidable piston 46 disposed in a main cylindrical section 48 of accumulator 42. Piston 46 is sealed to the interior of cylindrical section 48 by a bi-directional seal 49. An elongated tubular shaft 50 extends axially through piston 46 and is fixed to the piston. The interior of shaft 50 has a closed section 52 in the plane of piston 46. The piston also has an open (right) end 54. The opposite (left) end of shaft 50 is closed, although it can be open for a purpose described in detail below. The closed (left) end portion of shaft 50 extends outwardly through a fluid tight pressure seal 56 in the hydraulic end of main cylinder section 48. The open (right) ended section of shaft 50 extends through the pneumatic portion of the cylinder 48 into a reduceddiameter secondary hydraulic cylindrical section 58 of accumulator 42. The open end section of the shaft is slidably disposed in a fluid tight bi-directional pressure seal 60 seated at the junction between main cylinder section 48 and secondary hydraulic section 58. The inner end face of closed section 52 of shaft 50 and the annular end face of the shaft in the secondary hydraulic section of accumulator 42 are, in effect, a second piston coupled to piston 46. If piston 36 has a stroke of 20 feet, piston 46 desirably has a stroke range of about 25 feet; this assures that accumulator 42 will always be operated throughout the full stroke range of piston 36.

In use, piston 46 moves back and forth in accumulator main section 48 to accommodate the volume flow of hydraulic fluid to and from cylinder 34. Main section 48 on the left side, or slave side, of piston 46 is of equal or greater volume than cylinder 34 so that section 48 can act as a piston accumulator for the hydraulic fluid in cylinder 34.

An air bank 62 is formed by a series of compressed air bottles 64 and supplies air under pressure through a line 66 to the right side, or servo side, of piston 46. A compressor 68 supplies air to air bottles 64 through a line 70 which includes a pressure gauge 72; in this manner, the nominal charging air pressure in air bank 62, and in servo-accumulator 42, can be changed to suit the load applied to hook 38 which is to be maintained constant, within narrow limits, by system 35.

Air bank 62 is a component of a passive aspect of system 35 for resiliently establishing the magnitude of the load to be carried by hook 38 and for maintaining the established load constant within narrow limits. The passive aspect also includes a relatively small capacity pneumo-hydraulic accumulator 96 coupled between air bank 62 and servo-accumulator 42. The pneumatic end of accumulator 96 is connected to the air bank via an air duct 112 which includes a manually adjustable air pressure regulator 110. The hydraulic end of accumulator 96 is connected to a hydraulic line 92 which communicates to secondary hydraulic section 58 of servo-accumulator 42. Regulator 110 is adjusted so that the pressure in accumulator 96 is less than the nominal pressure charged into the air bank from compressor 68.

The opposite slave and servo surface areas of servo piston 46 per se are equal. However, the total servo fluid piston surface area of the servo accumulator 42 is greater than the slave fluid (the fluid in volume 37 below hook piston 36 in compensator cylinder 34) piston surface area of accumulator 42 by an amount equal to the sum of the area of the inner closed end surface of shaft 50 and the annular area of the shaft at its right end. This additional servo fluid piston surface area is communicated to the hydraulic end of accumulator 96 via hydraulic duct 92. In order that the steady-state loads on piston 36 and on ram 45 (the latter including shaft 50) may be balanced, the nominal charge pressure in air bank 62 and the regulated pressure in accumulator 96 are established in conjunction with their respective servo surface areas of ram 45 so that the total servo fluid forces applied to the ram are equal to the force applied to the ram by the slave hydraulic fluid when the desired load is carried by hook 38.

The term servo fluid" refers both to the air supplied to servo accumulator 42 via duct 66 from air bank 62 and to the hydraulic fluid supplied to the servo accumulator via duct 92.

The air and hydraulic servo surface areas of servo ram 45 are defined so that if the regulated pressure in accumulator 96 was zero and vessel 14 experienced an upward heave motion sufficient to move hook piston 36 into contact with the lower end of compensator cylinder 34, the air pressure in air bank 62 will not increase to more than the maximum pressure which is to be provided in volume 37 so as to maintain the hook load constant within the desired limits.

it may be noted at this point that known passive pneumo-hydraulic heave compensators include counterparts of elements 34, 36, 37, 44, 46, 48, 62 and 66 which have been described above. Elements 50, 58, 96 and 110 of system 35 are believed not to be present in any previously known purely passive heave compensators. Heave compensating system 35 is described hereinafter with reference to an arbitrary example pertinent to underwater coring operations. It is assumed that the desired load on coring tool is 20,000 lbs., and that the desired coring tool load is to be maintained constant to within plus or minus two percent of the 20,000 lbs. value. It is also assumed that the total weight of the drill string, of the coring tool, and of any other equipment suspended from hook 38 is 100,000 lbs. Accordingly, the hook is to support a constant load of 80,000 lbs. Thus, the maximum load to be carried by the coring tool is 20,400 lbs., and the minimum load on the coring tool is 19,600 lbs.

The operation of the passive air bank in maintaining a constant load on the coring tool will be understood from the following examples.

EXAMPLE I When the vessel heaves upward, displacement compensator cylinder 34, which is attached to the vessel via travelling block 26, moves upward with the vessel while hook piston 36 tends to stay stationary, i.e., to move downward in cylinder 34, which decreases the volume of hydraulic fluidbelow piston 36 in the compensator cylinder. The volume flow of fluid from cylinder 34 is taken up by the left side of servo accumulator 42, which causes servo piston 46 to move to the right in FIG. 2 against the bias of the passive air bank. The air bank contains a finite volume of gas, so movement of piston 46 to the right in servo accumulator 42 increases the air pressure on the right side of piston 46. As piston 46 continues to move farther to the right, air pressure in accumulator 42 continues to increase, which progressively reduces the ability of the passive air bank to accommodate the volume flow of fluid from cylinder 34. Thus, the load supported by the hook will tend to increase above its 80,000 lb. nominal level, and the load on the coring tool will tend to decrease below the desired 20,000 lb. load. However, as long as the upward heave amplitude of the vessel is relatively small, the buildup of-air pressure in accumulator 42 will have a tolerable effect on the accommodation provided by servo ram 45, and the load on the coring tool will stay within the desired minus two percent limit, i.e., the load on the tool will not reach the minimum 19,600 lbs. allowable load.

EXAMPLE [1 When the vessel heaves downward, displacement compensator cylinder 34 moves downward with the vessel, which tends to increase the hydraulic fluid volume below piston 36. The passive air charge in the air bank reacts by forcing servo ram piston 46 to the left to force hydraulic fluid into cylinder 34 below piston 36 to maintain piston 36 in a stationary position independently of the downward heave of the vessel. As piston 46 continues to move farther to the left in accumulator 42, the air pressure in the air bank will progressively drop, which will progressively reduce the ability of the passive air bank to help force hydraulic fluid into cylinder 34. Thus, the load supported by the hook will tend to decrease, and the load on the coring tool will tend to increase. However, as long as the amplitude of the downward heave is relatively small, the reduced air pressure in cylinder 42 will have a tolerable effect on the accommodation provided by servo ram 45, and the load on coring tool 20, will stay within the desired plus two percent limit, i.e., the load will not reach the maximum 20,400 lbs. allowable load.

Thus, for a small range of upward and downward heave excursions, the passive air bank will maintain the load variation at the coring tool within the desired plus or minus two percent limit. However, for large amplitude excursions, the passive air bank becomes more limited in its ability to maintain the load within its preestablished limits. The air bank is physically limited in its capability of responding to large amplitude excursions, because air bottles 64 and accumulator 42 contain only a finite volume of air. Thus, practical limitations on the allowable size of air bottles 64 and accumulator 42 limit the ability of the passive air bank to maintain the desired load limits for the entire envelope of heave excursions experienced by the vessel during normal underwater coring operations. In a typical situation the volume of accumulator 42 and air bottles 64, together with the charge pressure of the passive air bias, are design considerations which will control the load variations on the coring tool for a given range of heave excursions. For example, these parameters may result in a plus or minus two percent load variation on the coring tool for heave excursions within a range of, say, about sixty percent of the allowable heave excursions. For the remaining forty percent large-amplitude heave excursions servo ram 45 is assisted by an active hydraulic aspect (represented generally by reference numeral 74 in FIG. 2) of system 35 to maintain the desired plus or minus two percent load variation during heave excursions in which the passive aspect of system 35 loses its ability to maintain the desired load on the coring tool.

Active hydraulic sub-system 74 includes a hydraulic pump 76 operated by a motor 78. The pump has an intake line 80 for receiving hydraulic fluid from a reservoir 82. The pump forces the pumped fluid under pressure through a discharge line 84 to which is connected the hydraulic end of a pneumo-hydraulic accumulator 86. Hydraulic fluid in accumulator 86 is buffered by air pressure admitted to the pneumatic end of accumulator 86 through a line 87 directly from passive air bank 62. Accumulator 86 is provided in system 35 to absorb any shocks which may be produced on start-up or shutdown of pump 84.

A three-way servo valve 88 controls the flow of hy draulic fluid to and from secondary hydraulic section 58 of servo accumulator 42. Servo valve 88 includes an inlet port connected to the discharge side of pump 76 via a line 90. The three-way servo valve also includes suitable internal valving for allowing hydraulic fluid to flow through line 92 to or from secondary hydraulic section 58 of servo accumulator 42. Fluid flowing from accumulator 42 through line 92 is routed by servo valve 88 through a discharge line 94 leading to reservoir 82.

Servo valve 88 is operative to maintain one of three positions: (1) a supply position in which it allows hydraulic fluid to be fed by pump 76 through lines 90 and 92 to servo accumulator 42, (2) a discharge position in which it dumps hydraulic fluid from servo accumulator 42 through lines 92 and 94 into reservoir 82, and (3) a closed position in which there is no flow of hydraulic fluid to or from accumulator 42.

In use, servo valve 88 remains in its closed position as long as the passive air bank is capable of maintaining the load variation on the coring tool within the desired plus or minus two percent limits. As described above, the servo side of ram 45 is biased in part by the pressure of the hydraulic fluid in line 92 and in accumulator 96, which pressure is always admitted to the servo accumulator 42. Preferably, the bias produced by the passive air bank accounts for about ninety to ninety-five percent of the servo-fluid load on piston 46 during periods when the active aspect of system 35 is not in operation.

The operation of the active hydraulic sub-system 74 will be understood best with reference to the following examples:

EXAMPLE III During upward heave of the vessel, piston 46 moves to the right in accumulator 42 to accommodate the flow of fluid from cylinder 34. As described above in Example 1, this movement of the piston causes a buildup of pressure in the passive air bank. If the upward heave of the vessel is of sufficiently large amplitude that the passive air bank reaches the limit of its ability to maintain the force on coring tool 20 above the minimum desired load, servo valve 88 is automatically activated to move to its discharge position. This relieves the pressure provided by the hydraulic fluid acting on the servo side of piston 46, allowing the hydraulic fluid to flow from secondary hyraulic section 58 of the servo accumulator through lines 92 and 94 and into reservoir 82. This venting of hydraulic fluid to the reservoir produces a reduction in the servo fluid pressure applied to the servo piston and thereby actively assists the passive air bank in allowing servo ram piston 46 to move to the right in FIG. 2 to accommodate the volume flow of fluid from cylinder 34, while maintaining appropriate hydraulic pressure levels in the compensator cylinder, during the large amplitude upward heave of the vessel. Thus, piston 36 is able to remain in an essentially fixed position under essentially constant loading conditions independent of the large upward heave, which maintains the load on coring tool 20 above the desired minimum load limit.

There are several means for sensing when servo valve 88 should be activated to assist servo ram 45 in compensating for the upward or downward heave of the vessel. One such means is illustrated in FIG. 2 and includes a pressure sensor 98 in a hydraulic line 100 connected with the slave side of servo accumulator 42. The pressure of the fluid in line 100 is proportional to the load carried by hook 38, and therefore is proportional to the load on coring tool 20. Thus, the hydraulic pressure in cylinder 34 will be within a known range when the load variation on coring tool 20 is within its desired range. That is, a certain maximum and minimum pressure will be present in cylinder 34 when the load on tool 20 reaches its minimum and maximum allowable limits, respectively. Servo valve 88 is coupled to pressure sensor 98 via leads 102 to feed pressure information to servo valve 88. Thus, for example, when the sensed pressure in line 100 indicates that the force on the tool has reached or is closely approaching an allowable minimum load, servo valve 88 is automatically switches to its discharge position to dump fluid from the servo side of piston 46 to assist servo ram 45 in maintaining the load on the coring tool above the allowable minimum load.

Other means for sensing the load on coring tool 20 also may be used without departing from the scope of the invention. For example, hydraulic pressure on the servo side of servo ram 45 can be sensed to indirectly indicate the maximum or minimum allowable variation in the load on the coring tool.

The flow rate of fluid dumped back to reservoir 82 is controlled by a pilot-operated regulator 104 in line 94. A pilot 105 for regulator 104 is controlled by hydraulic fluid pressure in a line 106 coupled with line 100. A manually operated regulator 108 is provided in line 106 to control the desired flow rate of fluid to the reservoir. Regulator 104 maintains a gradual pressure drop on servo ram 45 to prevent a sudden impact from being exerted on the servo ram once the servo valve is activated to dump fluid to the reservoir. In certain applications regulator 104 can be replaced by a variable orifice.

Secondary accumulator 96 accommoda'tes'surges of hydraulic fluid associated with both the dumping of fluid from accumulator 42 and the flow of hydraulic fluid from pump 76 via servo valve 88. For example, when hydraulic fluid is vented to the reservoir as in Example III above, accumulator 96 controls the fluid pressure drop in the system so that the pressure decreases on a gradient curve, rather than in a linear fashion. The fluid pressure drop is controlled on a gradient so that the response of the compensation system will simulate the normal heave of the ship due to wave action which is also on a gradient, rather than reacting by suddenly dropping fluid pressure when the hydraulic assist is activated.

EXAMPLE IV During downward heave of the vessel, piston 46 moves to the left in servo accumulator 42 to force fluid into the bottom of cylinder 34. As described above in Example II, this movement of piston 46 causes a pressure reduction in the passive air bank. If the downward heave of the vessel is of sufficiently large amplitude that the passive air bank closely approaches or reaches the limit of its ability to maintain the force on coring tool 20 below the maximum desired load,, servo valve 88 is operated in response to a signal from pressure sensor 98 to move to its supply position in which pump 76 forces hydraulic fluid through lines 90 and 92 into servo accumulator 42. The fluid applies pressure to the servo side of piston 46 by way of shaft 50, which assists the passive air bank in moving the servo ram to the left in FIG. 2 to force fluid at the appropriate essentially constant pressure into the bottom of cylinder 34 and thereby maintain the load on the coring tool below the desired maximum load limit.

The flow rate of fluid from pump 76 through line 90 is controlled by a pilot-operated regulator 114. A pilot 115 of regulator 114 is controlled by hydraulic fluid pressure in a line 116 connected to line 100. A manually operated pressure regulator 118 is provided in line 116.

A secondary pneumo-hydraulic accumulator 120 also is connected to line 90 between regulator 114 and servo valve 88 to keep the fluid in line 90 stored at a regulated pressure. Accumulator 120 is buffered by air pressure from air bank 62 which is held at a regulated pressure by regulator 110. In use, accumulator 120 takes up surges in hydraulic fluid associated with the operation of regulator 114 and of servo valve 88.

When the hydraulic assist operates in the supply mode, as in Example IV above, accumulator 96 controls the fluid pressure increase in the system by taking on pressure so that pressure applied to the servo ram will increase on a gradient curve.

Thus, the heave compensator of this invention operates as a passive-active/passive system. Within narrow plus or minus limits, the heave compensation is initially accommodated by a passive pneumatic system (the bank of air bottles coupled to the compensator cylinder via the air/hydraulic servo accumulator). Compensation beyond these plus or minus limits is accommodated by an active hydraulic mechanism (including the hydraulic pump and appropriate sensors and controls) which acts in conjunction with the passive mechanism to compensate for the air pressure variations in the air bank as servo piston undergoes large displacements from its normal position in its cylinder.

The system shown in FIG. 2 also includes additional controls for use in providing improved response for the compensator. An onboard acceleromater 122 is coupled by leads 124 to servo valve 88 and provides a supplemental control for the servo valve. The accelerometer senses when an upward or downward heave acceleration is being applied to the vessel of sufficient magnitude that the passive air bank will not be able to maintain the load variation on the coring tool within the desired plus or minus two percent limits. When the accelerometer anticipates such a large heave excursion, servo valve 88 is activated to begin assisting the servo ram by the time the load on the coring tool approaches its plus or minus two percent limit.

In another supplemental control of system 35, the output signals of pressure transducer 98 are applied via leads 128 to a controller mechanism 126 for pump motor 78. The controller circuitry is arranged so that when the pressure transducer output signal is of a value indicative that the hydraulic fluid pressure in compensator cylinder 34 is ata value near its allowable maximum or minimum pressure value (say at 1.8 percent greater or less than the nominal hydraulic fluid pressure level corresponding to the specific load to be maintained on piston 36), the controller circuitry operates to commence operation of pump motor 78. In this manner, pump motor 78 is turned on in anticipation of the active aspect 74 of system 35 being required to assist the passive aspect of the system in maintaining the hydraulic fluid pressure in the compensator cylinder within its predetermined limits, i.e., plus or minus two percent. Motor controller 126 also includes a time delay circuit (not shown) which is operative to discontinue operation of motor 78 in the event that, within the period of the time delay circuit, the signal level from transducer 98 does not again reach a level indicating a plus of minus 1.8 percent variation from normal in the pressure of the hydraulic fluid in the compensator cylinder. Thus, if the pump motor is turned on by reason of an abnormal heave oscillation, as may be produced by an unusually high wave in an otherwise lowlevel wavetrain, the pump motor is not kept in operation. On the other hand, if the sea-state in the area adjacent the vessel is increasing in severity so that the vessel experiences more and more heave motions of a nature calling for the operation of the active aspect of system 35, operation of the pump motor is continued so that the active aspect of the system is available for instantaneous operation when required, and so that active assistance for the passive aspect of the system need not await start-up of the pump motor.

Shaft 50, carried by servo accumulator piston 46, is made hollow so that, when hydraulic fluid is applied to the accumulator from the active aspect of the system in the manner described above, the shaft is loaded predominantly in tension by virtue of the major portion of such hydraulic load being applied to piston 46 via the closed left end face of the shaft. The portion of the shaft lying to the right of piston 46 (as the wave accumulator is represented in FIG. 2) will be loaded in compression during these periods, but only at a relatively low level corresponding to the annular area of the shaft between its inner and outer diameters at its right end. It will be appreciated that the active hydraulic assistance to the passive air bank can be applied to piston 46 by the use of a separate piston in a separate cylinder, the separate piston being connected to servo piston 46 by a suitable solid shaft; in such a circumstance, the shaft interconnecting the two servo-systems may be loaded in compression such that the column properties of the shaft become significant. The hollow shaft arrangement illustrated in FIG. 2 minimizes column loading considerations, thereby enabling the use of a lightweight, small diameter, long stroke servo accumulator capable of being mounted conveniently in the structure of derrick 24. It is desirable that the servo accumulator be mounted as close to travelling block 26 as possible so that the length of flexible hydraulic conduit 44 can be minimized.

It will be apparent from the preceding description that the present heave compensator apparatus enables the load suspended from the hook encountered in the derrick of a floating drilling ship, for example, to be maintained constant during heaving motions of the vessel within much narrower limits than has been possible with the purely passive heave compensators known heretofore. This enhanced load-sensitive heave compensator is made possible by the use of an active mechanism, in conjunction with a prior passive compensator mechanism, to assist the passive mechanism in moving the servo accumulator piston to maintain the hydraulic pressure presented to the hook piston at or very close to the pressure level which corresponds to the hook load which is to be maintained constant.

Workers skilled in the art to which this invention pertains will readily appreciate that the heave compensator described above may be used to advantage in conjunction with operations in which some mechanism other than a hole-forming tool, such as a coring bit or a drill bit, is carried at the lower end of drill string 18. thus, the present heave compensator may be used to advantage where the mechanism carried by the lower end of the drill string is a blow-out preventer stack being landed upon a suitable wellhead structure at the appropriate stage during the formation of an underwater oil or gas well. The heave compensator has been described above with emphasis upon its constant load characteristics, but it will be apparent that these constant load characteristics are achieved in conjunction with a constant position capability of the compensator apparatus. Accordingly, in the case of the installation of a blow-out preventer stack, for example, the constant position characteristics of the compensator may be more meaningful than the constant load characteristics. The present heave compensator can be used to lower a blow-out preventer smoothly and controllably into mating engagement with a submerged well-head even though the vessel from which the blow-out preventer is suspended may be experiencing relatively high amplitude heaving motions.

The present invention has been described above with reference to a presently preferred embodiment thereof. Persons skilled in the art to which this invention pertains will readily appreciate that certain modifications and alterations in the illustrated and described preferred embodiment may be made while still relying upon and making use of the teachings of this invention. For example, instead of the linear-action compensator cylinder described and shown in combination with a string of drill pipe, a rotary-action mechanism can be substituted where the submerged article is suspended from a woven-wire cable. Also, it is not imperative that a linear-action compensator cylinder be vertically disposed as shown in the drawings. Moreover, it is readily appreciated that the hydraulic active sub-system illustrated in FIG. 2 applies a driving force to the servo ram only when compensation piston 36 is to be moved upward. The active assist sub-system vents hydraulic fluid acting on the servo ram. rather than applying a driving force to the ram, when the compensation piston is to be moved downward. However, instead of venting fluid from the servo ram, the system could be modified to apply an active driving force to the servo rarn when assisting downward movement of piston 36. In this instance, the left end of shaft 50 would be open to apply hydraulic fluid to piston 46, and three-way valve 88 would be replaced by a fourway servo valve having an additional hydraulic line leading to the left side of the servo ram. Accordingly, for these reasons and others, the foregoing description should not be considered as limiting the scope of this invention.

What is claimed is:

1. Motion compensating apparatus useful to maintain a substantially constant support force on an article suspended from a floating vessel by a vertical support member despite heaving movement and the like of the vessel in response to wave action and the like, the apparatus comprising:

displaceable means engagable between the vessel and the support member at the vessel and operable for varying the distance between the vessel and the article;

passive means coupled to the displaceable means for operating the displaceable means to establish in the support member a desired load corresponding to a desired value of support force for the article and to maintain the desired load constant within prese lected limits of variation thereof; and

active assist means coupled to the displaceable means and responsive to predetermined excursions in the desired load for actively and additively assisting the passive means to maintain the desired load within said preselected limits.

2. Apparatus according to claim 1 in which the displaceable means includes hydraulic means for supporting the article via a volume of hydraulic fluid which is adjustable in response to heave of the vessel to compensate for the displacement of the vessel, in which the passive means includes pneumatic means for applying a resilient bias load to the adjustable volume of hydraulic fluid, and in which the active means includes hydraulic means for applying an adjustable amount of hydraulic pressure to said volume in accordance with load variations in the support member to assist the pneumatic means in applying its bias load on the volume of hydraulic fluid so as to maintain the support force on the support member within preselected limits.

3. Motion compensating apparatus useful to maintain a substantially constant support force on an article suspended from a floating vessel by a vertical support member despite heaving movement and the like of the vessel in response to wave action and the like, the apparatus comprising:

hydraulic displaceable means engageable between the vessel and the support member at the vessel and operable for varying the distance between the vessel and the article, the displaceable means being adapted for supporting the article via a volume of hydraulic fluid which is adjustable in response to hcavc of the .vessel to compensate .for the displacement of the vessel;

passive means coupled to the displaceable means for operating the same to establish in the support member a desired load corresponding to a desired value of support force for the article and to maintain the desired load constant within preselected limits of variation thereof, the passive means including a pneumatic accumulator coupled to said volume of hydraulic fluid for accommodating flow of hydraulic fluid to and from said volume, a slidable piston disposed in the accumulator and movable in response to the volume flow of the hydraulic fluid, the piston having a slave side to which the hydraulic fluid is applied and an opposite servo side, and pneumatic means for applying a resilient bias load to the servo side of the piston and thereby to the adjustable volume of hydraulic fluid; and

active assist means coupled to the displaceable means and responsive to predetermined excursions in the desired load for actively assisting the passive means to maintain the desired load within said preselected limits, the active means including hydraulic means for applying an adjustable amount of hydraulic pressure to the servo side of the accumulator piston in accordance with load variations in the support member to assist the pneumatic means in applying its resilient bias load on the volume of hydraulic fluid, via the piston, so as to maintain the support force on the support member within the preselected limits.

4. Apparatus according to claim 3 in which the active means further includes means for sensing when the load in the support member approaches a maximum or minimum established by said preselected limits, and a servo system for decreasing the hydraulic pressure on the servo side of the piston when the sensed load ap proaches said minimum, and for increasing the hydraulic pressure on the servo side of the piston when the sensed load approaches said maximum.

5. Apparatus according to claim 4 in which the load sensing means senses the instantaneous hydraulic pressure in said adjustable volume.

6. Apparatus according to claim 4 including acceleration-sensitive means for sensing heave loading of the vessel and for operating the servo system to decrease or increase the hydraulic pressure applied to the servo side of the piston.

7. Apparatus according to claim 4 in which the servo system includes a pump and means for initiating operation of the pump in response to excursions in the sensed load from said desired load being a substantial portion of said maximum or minimum relative to said desired load.

8. A motion compensating apparatus useful to maintain a substantially constant support force on an article suspended from a floating vessel by a vertical support member despite heaving movement and the like of the vessel in response to wave action and the like, the apparatus comprising:

displaceable means comprising a compensator piston adapted to be connected to the support member and reciprocal in a compensator cylinder adapted to be connected to the vessel. and a quantity of slave hydraulic fluid at a selected pressure in the compensator cylinder for supporting the compensator piston and flowable out of and into the compensator cylinder in response to movement of the compensator piston;

passive means coupled to the displaceable means for operating the displaceable means to establish in the support member a desired load corresponding to the desired value of support force for the article and to maintain the desired load constant within preselected limits of variation thereof, the passive means including a pneumo-hydraulic accumulator cylinder having a reciprocable piston therein separating a slave-hydraulic volume in fluid flow communication with the fluid in the compensator cylinder from a servo air volume, and a volume of compressed air at a pressure related to the selected pressure and connected to the servo air volume in the accumulator cylinder; and

active assist means coupled to the displaceable means and responsive to predetermined excursions in the desired load for actively assisting the passive means to maintain the desired load within said predetermined limits, the active assist means including auxiliary piston means connected to the accumulator piston, and means for supplying and withdrawing hydraulic fluid to and from the auxiliary piston means in response to preselected changes in the slave hydraulic fluid pressure so as to assist the accumulator piston in moving to accommodate flow of slave hydraulic fluid.

9. Apparatus according to claim 8 wherein the means for supplying and withdrawing hydraulic fluid includes hydraulic pump and valve means coupled to the auxiliary piston, and means responsive to slave fluid pressure for controlling operation of the pump and valve means.

10. Apparatus according to claim 8 including a second pneumo-hydraulic accumulator continuously connected between the volume of compressed air and the auxiliary piston means.

11. Apparatus according to claim 10 including pressure regulating means connected between the volume of compressed air and the second accumulator.

12. Apparatus according to claim 9 wherein the pump and valve means are operable to withdraw hydraulic fluid from the auxiliary piston in response to a preselected increase in the slave hydraulic fluid pressure from said selected pressure thereof.

13. Motion compensating apparatus useful to maintain a substantially constant support force on an article supported from a relatively movable platform such as a floating vessel subject to movement by wave action and the like, the apparatus comprising:

displaceable means operable bidirectionally between spaced displacement limits and engageable between the platform and the article for carriage of the article and operable for varying the distance between the platform and the article;

passive bias means continuously coupled to the displaceable means for operating the displaceable means to establish on the displaceable means a desired load corresponding to a state of the displaceable means between said displacement limits; and servo means including active means for applying a variable bias to the displaceable means arithmetically additive to the bias applied by the passive bias means, and means for effectively coupling the active means to the displaceable means and for adjusting the variable bias applied by the active means in response to the nature and amount of variation in the desired load to cause the displaceable means to tend toward said state.

14. Apparatus according to claim 13 wherein the passive bias means is operative for compensating for low magnitude variations in the desired load occuring within predetermined load limits associated with the passive bias means, and wherein the servo means includes sensing means for sensing variations in the desired load beyond the predetermined load limits, and means responsive to the sensing means for operating the active means to compensate for beyond-limit variations in the desired load.

15. Apparatus according to claim 14 wherein the displaceable means is a variable volume hydraulic device having a pneumo-hydraulic servo ram coupled thereto, the passive bias means comprises means for applying a pneumatic bias to the pneumatic portion of the servo ram, and the active means comprises a hydraulic pump having its output applicable to the hydraulic portion of 'bi-directionally movable connection attachable between a fixed support on a floating platform and the load to be supported, said passive system being capable of being set to exert continuously on said connection a desired load-supporting force, and an active force modifying system operative to sense a deviation from said desired load-supporting force to arithmetically assist directly the passive system to cause said deviation to tend toward zero.

18. Motion compensating apparatus useful, in suspending an article from a floating vessel and the like, to compensate for vertical motion of the vessel to enable constant position or constant force suspension of the article, the apparatus comprising:

fluid powered displaceable means engageable between the vessel and the article and operable for varying the distance between the vessel and the article,

a piston accumulator coupled to the displaceable means for biasing the fluid in the displaceable means,

a passive system continuously coupled to the accumulator for continuous application to the piston of a characteristic predetermined bias load which normally varies in response to operation of the displaceable means, and

an active system coupled to the accumulator for application to the piston, separate from the passive bias load, of a variable bias load, the active system including means for sensing the pressure of said fluid and for adjusting the variable bias load so that the composite of the passive and variable bias loads on the piston is essentially constant during operation of the displaceable means. 

1. Motion compensating apparatus useful to maintain a substantially constant support force on an article suspended from a floating vessel by a vertical support member despite heaving movement and the like of the vessel in response to wave action and the like, the apparatus comprising: displaceable means engagable between the vessel and the support member at the vessel and operable for varying the distance between the vessel and the article; passive means coupled to the displaceable means for operating the displaceable means to establish in the support member a desired load corresponding to a desired value of support force for the article and to maintain the desired load constant within preselected limits of variation thereof; and active assist means coupled to the displaceable means and responsive to predetermined excursions in the desired load for actively and additively assisting the passive means to maintain the desired load within said preselected limits.
 2. Apparatus according to claim 1 in which the displaceable means includes hydraulic means for supporting the article via a volume of hydraulic fluid which is adjustable in response to heave of the vessel to compensate for the displacement of the vessel, in which the passive means includes pneumatic means for applying a resilient bias load to the adjustable volume of hydraulic fluid, and in which the active means includes hydraulic means for applying an adjustable amount of hydraulic pressure to said volume in accordance with load variations in the support member to assist the pneumatic means in applying its bias load on the volume of hydraulic fluid so as to maintain the support force on the support member within preselected limits.
 3. Motion compensating apparatus useful to maintain a substantially constant support force on an article suspended from a floating vessel by a vertical support member despite heaving movement and the like of the vessel in response to wave action and the like, the apparatus comprising: hydraulic displaceable means engageable between the vessel and the support member at the vessel and operable for varying the distance between the vessel and the article, the displaceable means being adapted for supporting the article via a volume of hydraulic fluid which is adjustable in response to heave of the vessel to compensate for the displacement of the vessel; passive means coupled to the displaceable means for operating the same to establish in the support member a desired load corresponding to a desired value of support force for the article and to maintain the desired load constant within preselected limits of variation thereof, the passive means including a pneumatic accumulator coupled to said volume of hydraulic fluid for accommodating flow of hydraulic fluid to and froM said volume, a slidable piston disposed in the accumulator and movable in response to the volume flow of the hydraulic fluid, the piston having a slave side to which the hydraulic fluid is applied and an opposite servo side, and pneumatic means for applying a resilient bias load to the servo side of the piston and thereby to the adjustable volume of hydraulic fluid; and active assist means coupled to the displaceable means and responsive to predetermined excursions in the desired load for actively assisting the passive means to maintain the desired load within said preselected limits, the active means including hydraulic means for applying an adjustable amount of hydraulic pressure to the servo side of the accumulator piston in accordance with load variations in the support member to assist the pneumatic means in applying its resilient bias load on the volume of hydraulic fluid, via the piston, so as to maintain the support force on the support member within the preselected limits.
 4. Apparatus according to claim 3 in which the active means further includes means for sensing when the load in the support member approaches a maximum or minimum established by said preselected limits, and a servo system for decreasing the hydraulic pressure on the servo side of the piston when the sensed load approaches said minimum, and for increasing the hydraulic pressure on the servo side of the piston when the sensed load approaches said maximum.
 5. Apparatus according to claim 4 in which the load sensing means senses the instantaneous hydraulic pressure in said adjustable volume.
 6. Apparatus according to claim 4 including acceleration-sensitive means for sensing heave loading of the vessel and for operating the servo system to decrease or increase the hydraulic pressure applied to the servo side of the piston.
 7. Apparatus according to claim 4 in which the servo system includes a pump and means for initiating operation of the pump in response to excursions in the sensed load from said desired load being a substantial portion of said maximum or minimum relative to said desired load.
 8. A motion compensating apparatus useful to maintain a substantially constant support force on an article suspended from a floating vessel by a vertical support member despite heaving movement and the like of the vessel in response to wave action and the like, the apparatus comprising: displaceable means comprising a compensator piston adapted to be connected to the support member and reciprocal in a compensator cylinder adapted to be connected to the vessel, and a quantity of slave hydraulic fluid at a selected pressure in the compensator cylinder for supporting the compensator piston and flowable out of and into the compensator cylinder in response to movement of the compensator piston; passive means coupled to the displaceable means for operating the displaceable means to establish in the support member a desired load corresponding to the desired value of support force for the article and to maintain the desired load constant within preselected limits of variation thereof, the passive means including a pneumo-hydraulic accumulator cylinder having a reciprocable piston therein separating a slave hydraulic volume in fluid flow communication with the fluid in the compensator cylinder from a servo air volume, and a volume of compressed air at a pressure related to the selected pressure and connected to the servo air volume in the accumulator cylinder; and active assist means coupled to the displaceable means and responsive to predetermined excursions in the desired load for actively assisting the passive means to maintain the desired load within said predetermined limits, the active assist means including auxiliary piston means connected to the accumulator piston, and means for supplying and withdrawing hydraulic fluid to and from the auxiliary piston means in response to preselected changes in the slave hydraulic fluid pressure so as to assist the accumulator Piston in moving to accommodate flow of slave hydraulic fluid.
 9. Apparatus according to claim 8 wherein the means for supplying and withdrawing hydraulic fluid includes hydraulic pump and valve means coupled to the auxiliary piston, and means responsive to slave fluid pressure for controlling operation of the pump and valve means.
 10. Apparatus according to claim 8 including a second pneumo-hydraulic accumulator continuously connected between the volume of compressed air and the auxiliary piston means.
 11. Apparatus according to claim 10 including pressure regulating means connected between the volume of compressed air and the second accumulator.
 12. Apparatus according to claim 9 wherein the pump and valve means are operable to withdraw hydraulic fluid from the auxiliary piston in response to a preselected increase in the slave hydraulic fluid pressure from said selected pressure thereof.
 13. Motion compensating apparatus useful to maintain a substantially constant support force on an article supported from a relatively movable platform such as a floating vessel subject to movement by wave action and the like, the apparatus comprising: displaceable means operable bidirectionally between spaced displacement limits and engageable between the platform and the article for carriage of the article and operable for varying the distance between the platform and the article; passive bias means continuously coupled to the displaceable means for operating the displaceable means to establish on the displaceable means a desired load corresponding to a state of the displaceable means between said displacement limits; and servo means including active means for applying a variable bias to the displaceable means arithmetically additive to the bias applied by the passive bias means, and means for effectively coupling the active means to the displaceable means and for adjusting the variable bias applied by the active means in response to the nature and amount of variation in the desired load to cause the displaceable means to tend toward said state.
 14. Apparatus according to claim 13 wherein the passive bias means is operative for compensating for low magnitude variations in the desired load occuring within predetermined load limits associated with the passive bias means, and wherein the servo means includes sensing means for sensing variations in the desired load beyond the predetermined load limits, and means responsive to the sensing means for operating the active means to compensate for beyond-limit variations in the desired load.
 15. Apparatus according to claim 14 wherein the displaceable means is a variable volume hydraulic device having a pneumo-hydraulic servo ram coupled thereto, the passive bias means comprises means for applying a pneumatic bias to the pneumatic portion of the servo ram, and the active means comprises a hydraulic pump having its output applicable to the hydraulic portion of the servo ram when the desired force varies beyond one of said predetermined limits.
 16. Apparatus according to claim 15 wherein the servo means includes means for relieving hydraulic fluid from the hydraulic portion of the servo ram when the desired force varies beyond the other of said predetermined limits.
 17. A heave compensating device for maintaining a load suspended from a floating platform at a substantially constant level comprising a passive load-supporting system including a resilient load-supparting bi-directionally movable connection attachable between a fixed support on a floating platform and the load to be supported, said passive system being capable of being set to exert continuously on said connection a desired load-supporting force, and an active force modifying system operative to sense a deviation from said desired loadsupporting force to arithmetically assist directly the passive system to cause said deviation to tend toward zero.
 18. Motion compensating apparatus useful, in suspending an article fRom a floating vessel and the like, to compensate for vertical motion of the vessel to enable constant position or constant force suspension of the article, the apparatus comprising: fluid powered displaceable means engageable between the vessel and the article and operable for varying the distance between the vessel and the article, a piston accumulator coupled to the displaceable means for biasing the fluid in the displaceable means, a passive system continuously coupled to the accumulator for continuous application to the piston of a characteristic predetermined bias load which normally varies in response to operation of the displaceable means, and an active system coupled to the accumulator for application to the piston, separate from the passive bias load, of a variable bias load, the active system including means for sensing the pressure of said fluid and for adjusting the variable bias load so that the composite of the passive and variable bias loads on the piston is essentially constant during operation of the displaceable means. 